Pituitary prolactin (PRL) cells from F-344 rats, both normal and mammary tumor bearing, are being studied in vitro. The presence of the 13762 mammary tumor suppresses pituitary PRL and GH function. This suppression is reversible after surgery or chemotherapy. Addition of rat serum (RS) to cells in culture markedly suppresses PRL release. Evidence to date shows that the inhibitory material is associated with proteins of 120,000 and 30,000 MW. Efforts are underway to develop a serum-free culture system for PRL cells. Finally, we have developed a method for the testing of dispersed PRL cells in vivo. This involves encapsulation of cells in Amicon Hollow Fibers prior to their implantation in certain ectopic sites (brain, femur, kidney capsule). After 1-14 days the capsules are removed and studied in vitro for PRL release. Results indicate that placement site markedly affects subsequent secretory function.